Aircraft brake systems have brake disk stacks comprised of interleaved rotor and stator disks. The disks are alternatively splined to the rotatable wheel and stationary (non-rotatable) torque tube. An end plate is provided at one end of the stack, while an axially moveable pressure plate is provided at the other end of the stack. Upon application of force to the pressure plate, the disks frictionally engage and provide braking activity to the vehicle.
Generally in the past, brakes have been assembled with the available wear portions of the rotors and stators having the same available wear thickness. These brakes were operated until all the wear portions of the disks were fully worn. At that time, the disks were removed and replaced with new unworn disks or refurbished or reworked disks. The heat sink mass of the brake is reduced by the total wear of all the disks in the brake stack. The heat sink mass must be above a certain level to hold the operating temperature of the brake down. Due to the size and weight limitations on aircraft, the thickness of available wear portions is also limited. The thickness of the available wear portions controls the number of landings between the replacement or refurbishing of the disks. The piston cylinder assemblies for actuating the brakes also have a length determined by piston travel, which is a function of the total wear of the disks. Where the brake is operated until all disks are fully worn, the piston travel is increased and accordingly the total brake envelope is increased as well as weight of the brake assembly. The brake envelope includes a heat sink envelope which is the distance between the end of the piston or thrust member facing the brake stack and the backing plate or reaction member.
Various different brake configurations have been proposed to minimize the turnaround time, maximize the brake cooling, reduce the amount of piston travel, as well as the size and the weight of the brake, while at the same time retaining a substantial portion of the heat sink mass to a lower operating temperature. For example, U.S. Pat. No. 3,480,115 to Lallemant discloses a brake, which comprises two groups of coaxial disks. The first group of coaxial disks is smooth since the disks are not equipped with a friction lining. The second group of disks has a friction lining. The disks of the first group are disposed alternatively with the disks of the second group. The disks of one group are angularly coupled to a rotatable structure such as an aircraft wheel and the disks of the other group are coupled to a non-rotating structure. Lallemant discloses various embodiments in which the disks of at least one of the two groups have thicknesses that vary from one disk to another depending upon the axial position of the disks. The thicker disks have greater thermal capacities due to their greater mass. According to Lallemant, the purpose of these various embodiments is to provide multiple disk brakes that fulfill the requirements of practice, particularly with respect to their longevity, uniformity of braking efficiency and conditions in which maintenance operations can be carried out. Furthermore, Lallemant contemplates the possibility of re-machining the smooth disks and moving these disks to a new axial position in the brake for which new axial position the reduced thickness that has been given to the smooth disks is adapted to the working conditions corresponding to the new position. Lallement alleges that the overall thermal capacity of the brake is not affected by the re-machining of the smooth disks and therefore, the efficiency of such brakes is constant.
U.S. Pat. No. 4,613,017 to Bok discloses a method of assembling and overhauling a disk brake having a plurality of disks with available wear portions of predetermined different thicknesses. The method comprises positioning first a first group of disks in overlapping relationship with a second group of disks. The first group of disks have an available wear portion of a first thickness and the second group of disks have an available wear portion of a second thickness which is greater than the thickness of the first group of disks. A third group of disks having a third thickness at an intermediate overhaul replace the first group of disks when the available wear portions of the first group of disks are substantially fully worn. The third group of disks have a third thickness which is greater than the thickness of each of the available wear portions of the second group of the disks at intermediate brake overhaul time.
Similarly, U.S. Pat. No. 4,742,895 to Bok discloses a carbon disk brake assembly. The assembly comprises a plurality of disks in which the first group of disks, for example, the stators and end plates, have an available wear portion of a first thickness which is less than and preferably one half of the thickness of the wear portions of the second group of disks, in this example, rotors. After a predetermined number of landings, the stators and end plates will be substantially fully worn. These worn stators and end plates are replaced by a third group of disks, which are new or refurbished stators and end plates. Preferably, the wear thickness of this third group of disks is double the thickness of the available wear portions of the rotors at the intermediate overhaul. The brake assembly is then operated to another intermediate overhaul when the available wear portions of the rotors will be fully worn and replaced by new or refurbished rotors.
U.S. Pat. No. 4,977,985 to Wells, et al. discloses a method of carrying out the maintenance of a multi-disk brake. The disks can be made of a carbon—carbon material. The brake comprises a stack of interleaved rotor and stator disks provided between a thrust member and a reaction member. The rotor and stator disks are selected and arranged so that the wear allowance of a set of disks at one end of the stack is less than the wear allowance of a set of disks at the other end of the stack. A fully worn group of disks is removed from one end of the stack during brake service operation following wear. The remaining partially worn disks are axially moved or shifted to a new position towards the one end of the stack. An unworn stack of disks is added at the other end of the stack.
Similarly, U.S. Pat. No. 5,323,880 to Wells, et al. discloses a multi-disk brake system. The brake comprises a stack of interleaved rotor and stator disks disposed in an axially aligned relationship. The disks are made of a carbon—carbon material, which provides the frictional surfaces of the disk as well as their structural integrity. The stack of disks comprises a first group of adjacent rotors and stators and a second group of adjacent rotors and stators in an axially aligned relationship with the first group. Each wear surface of the first group which confronts a wear surface of an adjacent disk of the first group is unworn. Each wear surface of a disk of the second group which confronts a wear surface of an adjacent surface of the second group is partly worn. Each group has an end disk, which confronts an end disk of the other group. The confronting wear surfaces of the end disks are either both unworn or are both partly worn.
U.S. Pat. No. 5,509,507 to Wells et al. discloses a multi-disk brake system for aircraft. This brake system comprises a stack of interleaved rotor and stator disks made of carbon—carbon material and disposed in an axially aligned relationship between a thrust device and a reaction member. The stack comprises a first group of adjacent rotor and stator disks and a second group of adjacent rotor and stator disks with the two groups in an axially aligned relationship. Only one disk of the first group contacts a disk of the second group. Each group has an end disk which confronts an end disk of the other group. The wear surfaces of the disks of the first group are thicker than the wear surfaces of each of the disks of the second group. At an intermediate overhaul time, after a predetermined number of brake applications, each wear surface of the first group which confronts a wear surface of an adjacent disk of the first group is only partly worn away, whereas each wear surface of a disk of the second group which confronts a wear surface of an adjacent disk of the second group is substantially fully worn away. Also at the intermediate overhaul, the confronting wear surfaces of the end disks are either both only partly worn away or both substantially worn away.
U.S. Pat. No. 5,295,560 to Moseley discloses a thermally balanced brake stack. The brake stack comprises a plurality of stator disks, a plurality of rotor disks interleaved with said stator disks, a pressure plate at one end of the stack and an end plate at the other end of the stack. The rotor and stator disks at the first and second end are thinner than the rotor disks at the center portion of the stack. According to the patent, this configuration minimizes the rate of temperature increase at the center of the stack while enabling quicker dissipation of heat at the end of the brake disk stack.
A pressure balanced brake stack is disclosed in U.S. Pat. No. 5,551,534 to Smithberger et al. The brake stack comprises a plurality of rotor disks, a plurality of stator disks interleaved with said rotor disks, a pressure plate at a first end of the stack and an end plate at the second end of the stack. The rotor disks at the first and second ends of the stack are thicker than the rotor disks adjacent thereto. Due to this configuration according to the patentee, the rotor disks at the first and second ends will deflect less and distribute pressure more uniformly throughout the stack.